WCDMA Phy Ch Info

8/12/2019 WCDMA Phy Ch Info

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1 26.01.2002 WCDMA Physical Layer

WCDMA Physical Layer


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Introduction

This lecture presents a general WCDMAor UTRA (UniversalTerrestrial Radio Access) FDD (Frequency Division Duplex)physical layer issues.

Spreading and Scrambling

Transport Channels

Physical Channels Signaling

Physical Layer Procedures

Mapping

to


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Some Parameters of WCDMA Physical Layer

Carrier Spacing 5 MHz (nominal)

Chip Rate 3.84 Mcps

Frame Length 10 ms (38400 chips)

No. of slots/frame 15

No. of chips/slot 2560 chips (Max. 2560 bits)

Uplink SF 4 to 256

Downlink SF 4 to 512

Channel Rate 7.5 Kbps to 960 Kbps


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Spreading and Scrambling


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Spreading Operation

Spreading means increasing the signal bandwidth

Strickly speaking, spreading includes two operations:

Channelisation (increases signal bandwidth) - using orthogonalcodes

Scrambling (does not affect the signal bandwidth) - using pseudo-noise codes

Data

channelization codes (SF) scrambling codes

bit rate chip rate chip rate


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Channelisation (1/3)

Channelisation codes are orthogonal codes, based on OrthogonalVariable Spreading Factor (OVSF) technique

The codes are fully orthogonal, i.e., they do not interfere with each

other, only if the codes are time synchronized

Thus, channelisation codes can separate the transmissions from asingle source

In the downlink, it can separate different users within one cell/sector Limited orthogonal codes must be reused in every cell

Problem: Interference if two cells use the same code

Solution: Scrambling codes to reduce inter-base-station interference


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In the uplink, it can only separate the physical channels/services of oneuser because the mobiles are not synchronised in time.

It is possible that two mobiles are using the same codes. In order to separate different users in the uplink, scrambling codes are

used.

The channelisation codes are picked from the code tree as shown innext slide.

One code tree is used with one scrambling code on top of the tree.

If c4,4is used, no codes from its subtree can be used (c8,7, c8,8, ).


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C ode tree

(c)

(c,c)

(c,-c)

. . .

c1,1=(1)

c2,1=(1,1)

c2,2=(1,-1)

c4,1=(1,1,1,1)

c4,2=(1,1,-1,-1)

c4,3=(1,-1,1,-1)

c4,4=(1,-1,-1,1)

. . .

SF=1 SF=2 SF=4 SF=8

c8,1

c8,2

c8,3

c8,4c8,5

c8,6c8,7

c8,8


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Scrambling

In the scrambling process the code sequence is multiplied with a

pseudorandom scrambling code.

The scrambling code can be a long code (a Gold code with 10 msperiod) or a short code (S(2) code).

In the downlink scrambling codes are used to reduce the inter-base-

station interference. Typically, each Node B has only one scramblingcode for UEs to separate base stations. Since a code tree under one

scrambling code is used by all users in its cell, proper code

management is needed.

In the uplink scrambling codes are used to separate the terminals.


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SummaryChannelisation code Scrambling code

Usage UL: Separation of physcial data

and control channels from same UE

DL: Seperation of different userswithin one cell

UL: Separation of terminals

DL: Separation of

cells/sectors

Length UL:4 256 chips same as SF

DL:4 512 chips same as SF

UL: 10ms=38400 chips

DL: 10ms=38499 chips

No. of

codes

No. of codes under one scrambling

code = SF

UL: Several millions

DL: 512

Code

family

Orthogonal Variable Spreading

Factor

Long 10ms code: Gold code

Short code: Extended S(2)

code family

Spreading Yes, increase transmission

bandwidth

No, does not affect

transmission bandwidth

Limited codes in eachcell for DL.

38400


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Transport Channels


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Channel Concepts

Three separate channels concepts in the UTRA: logical, transport, andphysical channels.

Logical channels define what type of data is transferred.

Transport channels define how and with which type of characteristics the

data is transferred by the physical layer. Physical data define the exact physical characteristics of the radio channel.

RLC layer

MAC layer

PHY layer

L2

L1

Logical Channel

Transport Channel

Physical Channel


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Transport Channels -> Physical Channels

(1/3) Transport channels contain the data generated at the higher layers,

which is carried over the air and are mapped in the physical layer todifferent physical channels.

The data is sent by transport block from MAC layer to physical layer

and generated by MAC layer every 10 ms.

The transport format of each transport channel is identified by the

Transport Format Indicator (TFI), which is used in the interlayercommunication between the MAC layer and physical layer.

Several transport channels can be multiplexed together by physical

layer to form a single Coded Composite Transport Channel(CCTrCh).


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Transport Channels -> Physical Channels

(2/3) The physical layer combines several TFI information into the

Transport Format Combination Indicator (TFCI), which indicate

which transport channels are active for the current frame.

Two types of transport channels: dedicatedchannels and commonchannels.

Dedicated channel reserved for a single user only.

Support fast power control and soft handover. Common channel can be used by any user at any time.

Dont support soft handover but some support fast power control.

In addition to the physical channels mapped from the transportchannels, there exist physical channels for signalingpurposes tocarry only information between network and the terminals.


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Transport Channels -> Physical Channels (3/3)

Common pilot channel CPICH

Collision detection/Channel assignment indicator channel

CD/CA-ICH

CPCH Status indication channel CSICH

Paging indication channel PICHAcquisition indication channel AICH

Synchronisation channel SCH

Signaling physical channels

Physical downlink shared channel PDSCH(DL) Downlink shared channelDSCH

Secondary common control physical channelS-CCPCH(DL) Forward access channel FACH

(DL) Paging channel PCH

Primary common control physical channelP-CCPCH(DL) Broadcast channel BCH

Physical common packet channelPCPCH(UL) Common packet channel CPCH

Physical random access channel PRACH(UL) Random access channel RACH

Dedicated physical data channel DPDCH

Dedicated physical control channelDPCCH

(UL/DL) Dedicated channel DCHPhysical ChannelTransport Channel


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UL Dedicated Channel DCH (1/3)

Due to audible interference to the audio equipment caused from thediscontinuous UL transmission, two dedicated physical channels are

I-Q/code multiplexing (called dual-channel QPSK modulation)

instead of time multiplexing.

Data (DPDCH) Data (DPDCH)DTX Period

Layer 1 Control Information (DPCCH)

*j

Data(DPDCH)

Control(DPDCH)

channelization code, cD

channelization code, cC

I+jQ

complex

scrambling code

BPSK for each channel


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Dedicated Physical Control Channel (DPCCH) has a fixed spreading factorof 256 and carries physical layer control information.

DPCCH has four fields: Pilot, TFCI, FBI, TPC.

Pilot channel estimation + SIR estimate for PC

TFCI bit rate, channel decoding, interleaving parameters for everyDPDCH frame

FBI (Feedback Information) transmission diversity in the DL

TPC (Transmission Power Control) power control command

DataDPDCH

DPCCH PILOT TFCI FBI TPC

0 21 14Uplink DPCH

2560 chips

10 ms


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Dedicated Physical Data Channel (DPDCH) has a spreading factorfrom 4 to 256 and its data rate may vary on a frame-by-frame basis.

Parallel channel codes can be used in order to provide 2 Mbps userdata

2.3 Mbps57404, with 6 parallel codes

480 kbps9604

240 kbps4808120 kbps24016

60 kbps12032

30 kbps6064

15 kbps30128

7.5 kbps15256

Max. user data rate with rate coding (approx.)

DPDCH channelbit rate (kbps)

DPDCH SF

3.84 Mcps/256=15 kbps


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UL Multiplexing and Channel Coding Chain

CRC attachment

TrBlk concatenation/

code blocksegmenation

Channel coding

Radio frameequalization

1stinterleaving

Radio framesegmentation

Rate matching

TrCH 1

. . .

TrCH multiplexing

PhyCH segmentation

2nd(intra-frame)interleaving

PhyCH mapping

DPDCH#1 DPDCH#2 DPDCH#N

TrCH 2

Other TrCHs

CCTrCh


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DL Dedicated Channel DCH (1/3)

In the DL no audible interference is generated with DTX because the

common channels are continuously transmitting.

Downlink DCH is transmitted on the Downlink Dedicated Physical

Channel (Downlink DPCH); thus, DPCCH and DPDCH are time-

multiplexed and using normal QPSK modulation.

DPDCH DPCCH

DATA TFCI DATA PILOT

0 21 14Downlink

DPCH

2560 chips

10 ms

TPC

DPCCH DPDCH DPCCH

Slot

No FBI


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A code tree under one scrambling code is shared by several users. Normally,one scrambling code and thus only one code tree is used per sector in the BS.

DCH SF does not vary on a frame-by-frame basis; thus, data rate is varied by

rate matching operation, puncturing or repeating bits, or with DTX, where thetransmission is off during part of the slot.

The SF is the same for all the codes with multicode transmission.

TrCh A TPC TrCh B PILOTTFCI

TrCh A TPC TrCh B PILOTTFCI DTX

Downlink DPCH slot

A full rate

A half rate


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UL DPDCH consists of BPSK symbols whereas DL DPDCH consists of QPSKsymbols. The bit rate in the DL DPDCH can be almost double that in the ULDPDCH.

2.3 Mbps5616576028804, with 3 parallel

codes

936 kbps187219209604

456 kbps9129604808215 kbps43248024016

105 kbps21024012032

45 kbps901206064

20-24 kbps42-516030128

6-12 kbps12-243015256

1-3 kbps3-6157.5512

Max. user data rate

with rate coding(approx.)

DPDCH channel

bit rate range(kbps)

Channel

bit rate(kbps)

Channel

symbol rate(kbps)

Spreading factor


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DL Multiplexing and Channel Coding Chain

CRC attachment

TrBlk concatenation/

code blocksegmenation

Channel coding

1stinsertion of DTXindication

1stinterleaving

Radio framesegmentation

Rate matching

TrCH 1

. . .

TrCH multiplexing

PhyCH segmentation

2nd(intra-frame)interleaving

PhyCH mapping

DPDCH#1 DPDCH#2 DPDCH#N

TrCH 2

Other TrCHs

CCTrCh

2nd insertion of DTXindication


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Downlink Shared Channel (DSCH)

Used for dedicated control or traffic data (bursty traffic).

Shared by several users. Each user may allocate a DSCH for a shortperiod of time based on a particular packet scheduling algorithm.

Support the use of fast power control, but not soft-handover.

Use a variable spreading factor on a frame-by-frame basis so that bitrate can be varied on a frame-by-frame basis.

Associated with a DL DPCH with the use of DPCCH. Such a DLDPCCH from TFCI provides the power control information, anindication to which terminal to decode the DSCH and spreading

code of the DSCH. Since the information of DSCH is provided from an associated DL

DPCH, the PDSCH frame may not be started before 3 slots after theend of that associated DL DPCH.


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Random Access Channel (RACH)

A contention-based uplink transport channel; thus, no scheduling isperformed.

Use of RACH

Carry control information from the UE to set up an initialconnection. For example, to register the UE after power-on to thenetwork or to perform location update or to initiate a call.

Send small amount of packet data to network for 1 to 2 frames.

Since it is needed to be heard from the whole cell for signalingpurposes, the data rate is quite low.

No power control is supported.


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RACH Operation

First, UE sends a preamble. The SF of the preamble is 256 and contain a signature sequence of 16

symbols a total length of 4096 chips.

Wait for the acknowledged with the Acquisition (AICH) from the BS.

In case no AICH received after a period of time, the UE sends anotherpreamble with higher power.

When AICH is received, UE sends 10 or 20 ms message part.

The SF for the message is from 32 to 256.

RACH Preamble AICH Preamble RACH Message

UE

BS


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Common Packet Channel (CPCH)

A contention-based uplink transport channel for transmission ofbursty data traffic.

Different from RACH, channel can be reserved for several frames

and it uses fast power control.

Information of CPCH is provided by

DL DPCCH for fast power control information.

Forward Access Channel (FACH) for higher layer DL signaling. CPCH operation is similar to RACH operation except that it has

Layer 1 Collision Detection (CD).

In RACH, one RACH message is lost, whereas in CPCH anundetected collision may lose several frames and cause extrainterference.


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CPCH Operation After receiving CPCH AICH,

UE sends a CPCH CD preamble with the same power from anothersignature.

If no collision after a certain time, the BS echo this signature back to theUE on the CD Indication Channel (CD-ICH).

Then, the UE sends data over several frames with fast power control.

The CPCH status indicator channel (CSICH) carries the status of differentCPCH information.

CPCH Preamble AICH Preamble CPCH Message

CPCH CD CPCH CD-ICH

UE

BS


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Broadcast Channel (BCH)

Downlink common transport channel.

The physical channel of BCH is Primary Common Control PhysicalChannel (Primary CCPCH).

BCH:

broadcast the system and cell-specific information, e.g., randomaccess codes or slots.

terminals must decode the broadcast channel to register to the cell.

uses high power in order to reach all users within a cell.


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Forward Access Channel (FACH)

Downlink common transport channel.

It can be multiplexed with PCH to the same physical channel,Secondary CCPCH, or standalone.

FACH:

carry control information to UEs within a cell.

carry small amount of packet data. no power control.

can have several FACHs. But the primary one must have low datarate in order to be received by all terminals.

In-band signaling is needed to inform for which user the data wasintended.


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Paging Channel (PCH)

Downlink common transport channel for transmission of paging and

notification messages, i.e., when the network wants to initiate

communication with the terminal.

It can be multiplexed with FACH to the same physical channel,

Secondary CCPCH, or standalone.

The identical paging message can be sent in a single cell or hundreds

of cells. The paging message has to be reached by all the terminals

within the whole cell.

Its transmission is associated with transmission of paging indicator in

paging indicator channel (PICH).


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Signaling Physical Channels


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Common Pilot Channel (CPICH)

Downlink channel with a fixed rate of 30 Kbps or SF of 256.

Scrambled with the cell-specific primary scrambling code.

Use for channel estimation reference at the terminal.

Two types: primary and secondary CPICH

Primary CPICH

the measurements for the handover and cell selection / reselection.

phase reference for SCH, primary CCPCH, AICH and etc.

Secondary CPICH may be phase reference for the secondaryCCPCH.

S h i ti Ch l (SCH) C ll


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Synchronisation Channel (SCH) CellSearching

SCH is used for cell search.

Two subchannels: primary and secondary SCH.

P-SCH and S-SCH are only sent during the first 256 chips of eachslot in parallel and time-multiplexed with the Primary CCPCH.

10 ms

0 1 14

0 1 142560 chips

256 chips

P-SCH

S-SCH

Synchronisation Channel (SCH) Cell


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Synchronisation Channel (SCH)CellSearching

Cell search using SCH has three basic steps:

The UE searches the 256-chip primary synchronisation code,which is common to all cells and is the same in every slot. Detect

peaks in the output of the filter corresponds to the slot boundary(slot synchronisation).

The UE seeks the largest peak secondary synchronisation code

(SSC). There are 64 unique SSC sequences. Each SSC sequencehas 15 SSCs. The UE needs to know 15 successive SSCs from theS-SCH, then it can determine the code group in order to know theframe boundary (frame synchronisation).

Each code group has 8 primary scrambling. The correct one isfound by each possible scrambling code in turn over the CPICHof that cell.

SSC Sequences


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SSC SequencesSecondary Synchronisation Code (SSC) and Code Group

:

:

61691213212313167796232

12131696167912331326231

41414916716491127115230

:

:

#14#13#12#11#10#9#8#7#6#5#4#3#2#1#0Codegroup

16 6 9 16 13 12 2 6 2 13 3 3 12 9 7 16 6 9 16 13 12

Received sequence of SSCs from S-SCHStart Frame


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Primary Common Control Physical Channel

(Primary CCPCH)

Carries broadcast channel (BCH).

Needs to be demodulated by all terminals within the cell.

Fixed rate of 30 kbps with a spreading factor of 256.

Contains no power control information. Primary CCPCH is time-multiplexed with SCH; thus, it does not use

the first 256 chips. Channel bit rate is reduced to 27 kbps.


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Secondary Common Control Physical Channel

(Secondary CCPCH)

Carries two transport channels: FACH and PCH, which can be

mapped to the same or separate channels.

Variable bit rate.

Fixed spreading factor is used. Data rate may vary with DTX or rate

matching parameters.

Contains no power control information.


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Physical Layer Procedures


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Power Control Procedure

BS

Fast Power Controlif SIRestimate


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Power Control (PC)(1/2)

FastClosed Loop PC Inner Loop PC Feedback information.

Uplink PC is used for near-far problem. Downlink PC is to ensurethat there is enough power for mobiles at the cell edge.

One PC command per slot 1500 Hz

Step 1 dB or 0.5 dB (1 PC command in every two slots).

The SIR target for fast closed loop PC is set by the outer loop PC.

Two special cases for fast closed loop PC: Soft handover: how to react to multiple power control commands

from several sources. At the mobile, a power downcommandhas higher priority over power upcommand.

Compressed mode: Large step size is used after a compressedframe to allow the power level to converge more quickly to thecorrect value after the break.

P C l (PC) (2/2)


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Power Control (PC)(2/2)

Closed Loop PC - Outer Loop PC

Set the SIR target in order to maintain a certain frame error rate(FER). Operated at radio network controller (RNC).

Open loop PC

No feedback information.

Make a rough estimate of the path loss by means of a downlinkbeacon signal.

Provide a coarse initial power setting of the mobile at thebeginning of a connection.

Apply only prior to initiating the transmission on RACH orCPCH.

T it Di it (BS) (1/2)


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Transmit Diversity (BS)(1/2)

Antenna diversity means that the same signal is transmitted orreceived via more than one antenna.

It can create multipath diversity against fading and shadowing.

Transmit diversity at the BS - open-loop and closed-loop.

Open Loop Mode

No feedback information from the UE to the BS.

BS decides the appropriate parameters for the TX diversity.

Normally use for common channels because feedbackinformation from a particular UE may not be good for othersusing the same common channel.

Uses space-time-block-coding-based transmit diversity (STTD).

T it Di it (BS) (2/2)


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Transmit Diversity (BS)(2/2)

Closed Loop Mode

Feedback information from the UE to the BS to optimize thetransmission from the diversity antenna.

Normally use for dedicated channels because they have thefeedback information bits (FBI).

Based on FBI, the BS can adjust the phase and/or amplitude of

the antennas.

C d M d (1/2)


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Compressed Mode (1/2)

The compressed mode is needed when making measurement fromanother frequency.

The transmission and reception are halted for a short time to perform

measurements on the other frequencies.

NormalFrame

NormalFrame

CompressedMode

Measurementgap



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Three methods for compressed mode:

Lowering the data rate from higher layers.

Increasing the data rate by changing the spreading factor. Reducing the symbol rate by puncturing at the physical layermultiplexing chain.

More power is needed during compressed mode.

No power control during compressed mode. Large step size is usedafter a compressed frame to allow the power level to converge morequickly to the correct value after the break.

Handover


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Handover

Intra-mode handover

Include soft handover, softer handover and hard handover.

Rely on the Ec/No measurement performed from the CPICH. Inter-mode handover

Handover to the UTRA TDD mode.

Inter-system handover Handover to other system, such as GSM.

Make measurement on the frequency during compressed mode.

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